Allow overriding context mappings in children elements

[dawagner]

When a context mapping key is defined in an element, the same key may not be defined again in a sub-element. This behaviour is useful to avoid a certain class of mistakes but the opposite is also sometimes useful when, in a large list of elements, a single elements stands out and has to override the context.

Example:

<ComponentType Name="T" Mapping="Key:Value">
...
</ComponentType>
...
<InstanceDefinition>
  <Component Name="1" Type="T"/>
  <Component Name="2" Type="T"/>
  <Component Name="3" Type="T"/>
  <Component Name="4" Type="T"/>
  <Component Name="5" Type="T"/>
  <Component Name="6" Type="T" Mapping="Key:SpecialValue"/>
</InstanceDefinition>